1. Field of the Invention
The present invention relates to data transmission techniques, and more particularly to data transmission techniques in which the data transmitted is structured in frames, each of which contains payload data preceded by a synchronization pattern serving to enable receiver equipment to be synchronized to the transmitter equipment.
2. Description of Related Art
In this type of transmission, it is necessary to make provisions for coping with the risks of the synchronization pattern being imitated by the payload data. It is thus common to insert bits among the payload bits to avoid such imitation: e.g., if the synchronization pattern is a sequence of eight xe2x80x9c0xe2x80x9d bits, then a xe2x80x9c1xe2x80x9d bit is inserted every seven bits.
However, it can be understood that the bits inserted in this way are inserted to the detriment of payload data bits, which raises a problem of frame efficiency in particular when, with data-rate remaining constant, it is desirable for increases to the payload transmitted to be possible.
A particular object of the present invention is to provide a solution to that problem.
Furthermore, Document EP 0 367 215 discloses a method which, besides any frame synchronization problem, prevents a string of sixteen xe2x80x9c0xe2x80x9d bits in a transmitted frame from disturbing bit clock recovery on reception, wherein the frame is subdivided into 8-bit time slots. In other words, the frame is analyzed by means of a fixed window of width 8 bits, and when sequences of eight 0 bits are detected, instead of transmitting such sequences, 8-bit sequences are transmitted that contain data identifying the time slots of the frame into which such sequences must be re-inserted on reception.
The present invention provides a data transmission frame containing payload data preceded by a synchronization pattern, said frame being essentially characterized in that it does not contain any payload data sequences that imitate said synchronization pattern, such sequences also being referred to as xe2x80x9cforbiddenxe2x80x9d sequences, and being replaced with xe2x80x9csubstitutexe2x80x9d data serving to enable the forbidden sequences to be re-inserted, on reception, among the received payload data.
Thus, since any payload data sequences that imitate said synchronization pattern are removed prior to transmission, it is no longer necessary to transmit bits at regular intervals to avoid such imitation, and, provided that the number of items of said substitute data does not exceed the number of items of payload data removed in this way, the efficiency of the frame can be improved.
According to another characteristic, said substitute data itself comprises two types of data, namely:
xe2x80x9coperatingxe2x80x9d data making it possible, on reception, to determine the extent to which forbidden sequences were present in the payload data to be transmitted; and
xe2x80x9caddressxe2x80x9d data making it possible, on reception, to determine more precisely the location(s) at which a forbidden sequence is to be inserted among the received payload data.
According to another characteristic, said substitute data itself comprises two types of data, namely:
xe2x80x9coperatingxe2x80x9d data making it possible, on reception, to determine the extent to which forbidden sequences were present in the payload data to be transmitted; and
xe2x80x9caddressxe2x80x9d data making it possible, on reception, to determine more precisely the location(s) at which a forbidden sequence is to be inserted among the received payload data.
Furthermore, in Document EP 0 367 215, since at the most 7 bits in a sequence of 8 bits are available for indicating that time slot into which a sequence of eight 0 bits must be re-inserted, the maximum number of time slots per frame is equal to 27 (i.e., 128), and so the maximum number of bits per frame is equal to 8xc3x9727 (i.e., 1024).
The present invention makes it possible, in particular, with the same number N of bits in a forbidden sequence, to increase the maximum number M of bits per frame.
Thus, according to another characteristic, the number of bits in a substitute sequence is equal to the number of bits in a forbidden sequence, and, according to another characteristic, with xe2x80x9cnxe2x80x9d bits out of the N bits of a substitute sequence being used to constitute said address data (where xe2x80x9cnxe2x80x9d is not more than Nxe2x88x921), the maximum number M of bits per frame is equal to 27.
Thus, by way of comparison with the above-mentioned document, with N equal to 16, and xe2x80x9cnxe2x80x9d equal to Nxe2x88x921, the number M obtained with the present invention is equal to 32,768, which is much higher than the 1024 bits obtained as defined in the above-;mentioned document.
The present invention also provides a method of transmitting such frames, making it possible to form such frames from an incident stream of payload data, said method being essentially characterized in that it comprises the following steps:
analyzing said incident stream to detect any payload data sequences that imitate said forbidden sequence;
computing substitute data to enable forbidden sequences to be re-inserted, on reception, among the received payload data; and
transmitting payload data that contains no forbidden sequences, and substitute data computed in this way, to form said frame.
According to another characteristic, the analyzing step is performed by means of a sliding window of width equal to N bits, where N designates the number of bits in a forbidden sequence.
The present invention also provides corresponding transmitter apparatus.
The present invention also provides a method of receiving such frames, making it possible to extract a stream of payload data from such frames received in this way.
The present invention also provides corresponding receiver equipment.
The present invention is particularly applicable to data transmission within the infrastructures of a radio-communications network for mobile stations, such as in particular the Global System for Mobile Communications (GSM) network. Payload data is conveyed in such infrastructures by means of such frames, each of which comprises payload data preceded by a synchronization pattern, in particular Transcoder/Rate Adapter Unit (TRAU) frames. The data rate of such frames thus sets the maximum data rate for the data-transmission services offered by the network, which maximum data rate is currently 9600 bits per second (bit/s) for 16-kbit/s TRATJ frames, or for frames obtained by multiplexing 8-kbit/s TRAU frames in pairs. The present invention thus makes it possible to increase the maximum data rate beyond current limits, land to transmit data at a data rate higher than 9600 bit/s, by making it possible to increase the payload conveyed by such 16-kbit/s TRAU frames or by such frames obtained by multiplexing such 8-bit/s TRAU frames in pairs.